Processes for embossing and for debossing and perforating heat-softened thermoplastic film are known. As used herein, a web of film is a thin sheet of thermoplastic material of definite, preferably substantially uniform, width and indefinite length. A web of embossed film is such a web of film in which numerous protuberances have been formed to provide it with a substantial third dimensional structure perpendicular to the plane of the film. A web of debossed and perforated film has protuberances which provide it a third dimensional structure perpendicular to the plane of the film in which many or all of the protuberances have holes. The protuberances of webs of embossed and of debossed and perforated film may be of a wide variety of sizes and shapes or may all be identical. Collectively such webs of embossed and debossed and perforated films are referred to herein as webs of formed film.
In processes for producing a web of formed film, a web of heat-softened film is provided on a patterned, perforated outer surface (referred to herein as a forming surface) of a structure such as an endless belt or a drum's cylindrical surface. The heat-softened film is generally pulled into conformity with the forming surface due to the presence of a vacuum on the opposite side of the forming structure and the holes in the forming surface. Whether the web of film is simply embossed or is debossed and perforated will depend on the size of the holes in the forming surface, the softness and thickness of the film being formed, the pressure differential across the film, and the time during which said pressure differential is applied.
Processes for vacuum embossing of webs of thermoplastic film are disclosed in U.S. Pat. No. Re. 23,910 issued to Smith & Smith on Dec. 12, 1954; U.S. Pat. Nos. 2,776,451 and 2,776,452 both issued to Chavannes on Jan. 8, 1957; and U.S. Pat. No. 2,905,969 issued to Gilbert & Prendergast on Sept. 29, 1959. Processes for the production of webs of debossed and perforated thermoplastic films are disclosed in U.S. Pat. No. 3,038,198 issued to Schaar on June 12, 1962; U.S. Pat. No. 3,054,148 issued to Zimmerli on Sept. 18, 1962; U.S. Pat. No. 4,151,240 issued to Lucas & Van Coney on Apr. 24, 1979; U.S. Pat. No. 4,155,693 issued to Raley on May 22, 1979; U.S. Pat. No. 4,259,286 issued to Louis, Sorensen & Ballard on Mar. 31, 1981; U.S. Pat. No. 4,317,792 issued to Raley & Adams on Mar. 2, 1982; U.S. Pat. No. 4,342,314 issued to Radel & Thompson on Aug. 3, 1982; and U.S. Pat. No. 4,395,215 issued to Bishop on July 26, 1983.
The processes disclosed in the references cited above require that the thermoplastic film be heat-softened in order to achieve the desired embossing or debossing and perforation of the film. This can be achieved as disclosed in many of the above references by heating an existing web of film above its melt temperature range such that it is in a molten state and will readily flow and attain a new configuration. Alternatively, the molten film may be achieved by feeding a web of film directly from a film extruder onto the forming surface. Such a process is disclosed in U.S. Pat. No. 3,685,930 issued to Davis & Elliot on Aug. 22, 1972, where a web of thermoplastic film is extruded directly onto the outer surface of an endless belt and a vacuum is pulled beneath the belt to make the molten web of film assume the configuration of the outer belt surface. Similarly, U.S. Pat. No. 3,709,647 issued to Barnhart on Jan. 9, 1973, discloses a web of molten thermoplastic film extruded directly onto the outer cylindrical surface of a vacuum forming drum.
In producing webs of embossed or debossed and perforated film on a patterned, perforated surface such that the film acquires a three-dimensional structure, it is generally desirable to cool the film so that its three-dimensional structure is set prior to removing the web of formed film from the forming surface. This makes the web of formed film much less susceptible to distortion of its protuberances by external forces.
A substantial amount of cooling of webs of formed film can occur by heat transfer from the film to the forming surface. In Davis and Elliott, such heat transfer is enhanced by cooling the endless belt after stripping the web of formed film from the belt and before a web of molten film is again applied to the belt. In Barnhart, a cooling medium is used inside the drum to cool the forming surface.
Cooling can also be assisted by contacting the outer film surface with an external cooling medium while the web of formed film is on the forming surface. For webs of debossed and perforated film, cooling of the formed film is usually assisted by drawing cooling air through the perforations with vacuum prior to removing the web of formed film from the forming surface; such a cooling process is disclosed, for example, in Lucas and Van Coney. Chavannes '452 discloses contacting a web of embossed film with a wet belt to cool the film. Gilbert and Prendergast discloses use of a water spray to cool a web of embossed film on the forming surface of an embossing drum.